The invention relates to semiconductor device manufacturing, and more particularly to scheduling photolithography in a wafer fab.
The manufacture of integrated circuit products typically involves hundreds of processing steps applied to a semiconductor (e.g., silicon) wafer over several weeks. Indeed, a typical wafer fab may at any time contain 50,000 wafers as work in progress and with a 20,000 wafer turnover per month. The basic processing operations in a wafer fab are photolithography, etching-polishing, deposition, implantation, oxidation, and diffusion-anneal plus various cleanings and inspections-measurements; and each of these processing operations has an associated set of tools. Now to yield multilevel-metal CMOS integrated circuits, a single wafer may by subject to 10-30 photolithography operations, 10-20 etching-polishing operations, 5-10 implantation operations, and so forth with these operations in multiple sequences such as photolithography-implant-anneal and deposition-photolithography-etch. Thus the sequence of processing operations (logpoints) for a single wafer is reentrant in that the same tool may be used multiple times during the wafer processing. And the problem is to schedule the wafers for processing operations, typically in lots of 25 wafers which all receive identical processing, on the tools in the fab to maximize utilization of the tools. Further, stochastic events such as tool breakdowns or varying lot priorities add to the scheduling complications.
A further complication of wafer fab scheduling arises from the simultaneous manufacture of several different integrated circuit products; each product has its own sequence of processing operations and requires its own set of reticles for use in photolithography operations. Thus despite a factory possessing several photolithography units (photoresist coaters, scanners-steppers for patterned photoresist exposure, exposed photoresist developers), the high cost of reticles and the limited reticle capacity of a scanner makes the photolithography units more like unique, mutually exclusive resources within a factory. And at any time perhaps 10% of the wafers in the fab (e.g., 5,000 wafers or 200 lots) are awaiting photolithography operation, and scheduling a particular lot at a particular logpoint for a particular scanner requires the corresponding reticle be loaded into the scanner.
Akcalt et al, Cycle-Time Improvements for Photolithography Process in Semiconductor Manufacturing, 14 IEEE Tran.Semi.Manuf. 48 (2001) describes simulations of a scheduling system with first-in/first-out dispatching.
However, the known wafer fab scheduling methods do not provide effective real time usage.